The present invention relates to an image server which allows the user to photograph a picture as the user performs remote control of a camera via a network such as the Internet and which transmits stable image data, and an image server system which connects the image server and a client terminal via a network.
In recent years, an image server is attracting public attention which allows the user to remotely control a camera via a network such as the Internet to obtain an image. Such an image server changes the exposure time when the subject darkens and switches from the normal exposure mode to the long exposure mode. In case the mode is switched from the normal exposure mode to the long exposure mode, control information for automatic control such as Auto Gain Control (hereinafter referred to as the AGC control), Auto Iris (hereinafter referred to as the AI), Auto White Balance (hereinafter referred to as the AWB), and Auto Focus (hereinafter referred to as the AF) cannot properly follow the change caused by extension of the exposure cycle. That is, in the normal exposure mode, the control information for AGC, AI, AWB and AF is obtained per frame in a predetermined cycle to perform automatic control and transmit an image data. In the long exposure mode, evaluation value information is fixed for a long cycle. This leaves a strong sense of incongruity when the normal exposure mode is switched to the long exposure mode. Moreover, such an image server cannot follow a rapid change in brightness, for example. In such a case, the client performs control only after the information, resulting in poor camera operability with a delay in response.
In order to prevent a time delay before a favorable output image is obtained due to a change in the incident light volume on entering the long exposure mode, imaging apparatus has been proposed which comprises means for decreasing the response speed of automatic signal volume control only in the long exposure mode (see Japanese Patent Laid-Open No. 305671/1989).
As mentioned hereinabove, a related art image server cannot follow a change caused by an extended exposure cycle. In the long exposure mode, evaluation value information is fixed for a long cycle, and the server has failed to follow for example a rapid change in brightness. Moreover, when the focus is dislocated, the out-of-focus state persists, an acceptable phenomenon. While the imaging apparatus according to Japanese Patent Laid-Open No. 305671/1989 prevents iris oscillation by delaying the response speed in order to follow the switching between exposure modes, particular disclosure is not found concerning the Auto Focus mechanism.
Among several methods for detection a focus position in the Auto Focus mode, a method for using the high-frequency component of a picture signal is often used by a network camera. FIG. 4 illustrates the principle of related art Auto Focus control. FIG. 5A shows the evaluation value level of Auto Focus and its control value of a related art image server in the normal exposure mode. FIG. 5B shows the evaluation value level of Auto Focus and its control value of the image server shown in FIG. 5A in the long exposure mode. In case a picture signal is used to detect a focus position, the high-frequency component of the picture signal shows its maximum value while the focus is achieved, as shown in FIG. 4. This is because a picture in focus has a steep contour, which means a rich high-frequency component is contained. From this characteristic, the focus position is at the maximum value of the hill of the high-frequency component or where the gradient is 0.
The related art AF control which achieves focus by using this characteristic is shown in FIGS. 5A and 5B. The evaluation value of focus used in this example is a high-frequency component extracted from the picture signal mentioned above. When the subject is out of focus, the level of the focus evaluation value is low. As the subjects gradually becomes in focus, the level rises and reaches a maximum value when the focus is achieved. Thus, in the related art, a focus lens travels in the direction where the level of the evaluation value increases and a position where the maximum value is obtained is determined as a focus position.
In pursuit of a focus position, the focus lens travels in equal distances in accordance with the control value. The term “equal distances” is used for simplicity and the real control procedure will be given later. For the long exposure time, an instruction for movement is issued per 1V (timing period of vertical synchronizing signal). The focus lens travels in equal distances in the direction where the level of the focus evaluation value rises. As shown in FIG. 5A, the focus evaluation value rises with a change in control value (travel of the focus lens) and reaches a maximum value at a point. Then the evaluation value level drops at time point T7. This shows that the focus evaluation value has dropped after it has peaked. This trend indicates that the control value (travel position of focus lens) immediately preceding the point where the focus evaluation value has dropped can be employed as a focus control value. Thus the focus lens is set to this value.
In the long exposure mode, the cycle where a picture signal is obtained is extended with the extension of the exposure period, and accordingly the signal level detection cycle is extended, and the cycle where the focus lens position is set is extended as a matter of fact. All this phenomena result in a slower response. While FIG. 5B shows a double exposure time where the exposure time is double the normal exposure time, the response time until focusing exceeds double that in normal exposure as the period until the focus evaluation value reaches a maximum value (time until t7*) exceeds double that in the normal exposure. In the related art image server and image server system, position setting of the focus lens is extended while in the long exposure mode. This takes time in focus control, with poor responsivity.
While the focus lens travels in equal distances in the foregoing example, focus evaluation values of various magnitudes are obtained so that the travel volume is controlled with relation to the gradient in order to accommodate this phenomenon. A gradient is detected at the start of control. When the gradient is larger than a predetermined value, the dislocation of focus is assumed to be large so that control is made using a large travel volume. In case the gradient is smaller than the predetermined value, the current position is near a peak so that control is made using a small travel volume. The former indicates a hill-climbing phase to approach a peak while the latter is a phase for checking the peak position. The travel volume is changed in accordance with the gradient in a phase also.
Assuming that the motor for driving a focus lens is a stepping motor, the focus lens travels while being rotated in 16 steps, 12 steps or 8 steps in accordance with the gradient in a hill-climbing phase with steep gradient. In a peak-checking phase with mild gradient, the focus lens travels while being rotated in 4 steps, 2 steps or 1 step in accordance with the gradient. Focusing takes time when in the long exposure mode even in case the travel volume is changed between phases. Thus the responsivity remains poor.
Further, as mentioned above, an image server according to the related art transmits to a client an image of a subject which gives a visual sense of incongruity due to a sudden increase in the brightness of the subject during switchover between exposure modes. In this way, the image server and the image server system according to the related art has no other choice than transmit am image which gives a visual sense of incongruity for example when the system has entered the long exposure mode.